Petrol retailers have kept prices at the pump high despite cheaper crude oil.
The Australian Competition and Consumer Commission's fourth quarterly report into the petroleum industry found retail margins are at their highest level since monitoring began in 2002.
"We will be closely monitoring gross retail margins in the coming months, because high retail margins likely indicate increased profits of the petrol companies at the expense of motorists," the consumer watchdog said on Tuesday.
ACCC chairman Rod Sims said part of the reason why retail prices did not fall by as much as international prices was the weaker Australian dollar.
Had the dollar been at the same level as it was in January 2013 ($US1.05) retail petrol prices in the quarter would have been around 20 cents a litre cheaper.
Sydney and Brisbane had the highest gross retail margins at around 14 cents a litre - six cents higher than in Melbourne.
The average retail petrol price in Sydney, Melbourne, Brisbane, Adelaide and Perth was 133.2 cents a litre - down 2.6 cents a litre from the June quarter.
Prices remained higher in regional areas due to less retail competition, lower fuel volumes sold, distance and lower convenience store sales.
While monthly average prices in the five largest cities fell by 11.3 cents a litre over the September quarter, regional prices fell by only 6.7 cents a litre - averaging 134.6.
NRMA spokesman Peter Khoury said profit margins had been as high as 26 cents a litre in some cities.
"Not only are oil companies making more profit at the high end but major players are pushing prices up at the lower end of the cycle," he told AAP.
"The forecast for prices as a result of international factors is in the positive for motorists, but this report shows we are still paying more at the pump because of these profit margins."
Prices on average needed to fall further and extra scrutiny of fuel companies would be vital to ensure they flow through to motorists, Mr Khoury said.
MONTHLY AVERAGE PETROL PRICES - JUNE AND SEPTEMBER QUARTERS 2015
Sydney - June 140.1, Sept 130.1
Melbourne - June 139.3, Sept 127.6
Brisbane - June 144.6, Sept 130.6
Adelaide - June 140.6, Sept 129.0
Perth - June 138.2, Sept 129.3
Hobart - June 143.1, Sept 137.8
Canberra - June 144.5, Sept 132.8
Darwin - June 134.9, Sept 130.4
(Source: ACCC)
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